This invention relates to a manufacturing process of high strength and tough steel bar, rod and wire, (hereinafter briefly referred to as wire) and the process for producing the same.
Increase in total reduction in area of drawing or increase in the strength of raw material is generally adopted in order to attain high strength steel wire. In case the total reduction in area is increased to attain higher strength wire, however, the toughness is sharply lowered when the strength of wires reaches the area shaded in FIG. 12. In other words delamination takes place at torsion test. As the bending property also deteriorates, it can also cause breakage of ropes, aluminium cables steel reinforced and PC strand at the stage of stranding or closing, breakage at the stage of forming spring, or breakage of wire in the middle of drawing.
Cr has been used increase the strength of raw material after patenting. Addition of Cr, however, increases smut at the pickling process before drawing. Productivity and efficiency in the drawing process is lowered due to longer pickling time and defective lubrication film caused by smut.
In order to attain the plated high carbon hard drawn steel wire or piano wire as specified in Japan Industrial Standard (JIS), it is necessary to increase strength of the steel wire before plating as the strength is greatly lowered by galvanizing.
According to JIS, high carbon steel wire is specified by diameter and tensile strength, for example hard drawn steel wire is specified by the tensile strength of 220 kgf/mm.sup.2 or higher for 1.0 mm diameter and smaller, and by over 200 kgf/mm.sup.2 for 2.5 mm diameter and smaller. Where the diameter is over 3.5 mm, however, 210 kgf/mm.sup.2 can hardly be attained even with piano wire. This is because the torsion value of wire with the diameter of 3.5 mm and over is diminished to an abnormal level when tensile strength of piano wire exceeds 220 kgf/mm.sup.2 or delamination takes place in torsion test, is higher deformation to attain the tensile strength exceeding (240-68 log d) kgf/mm.sup.2 and it makes the manufacturing difficult. For hard drawn steel wires of lower grade, in particular, it is very hard to maintain high toughness with the strength of over 210 kgf/mm.sup.2 for the wire with the diameter of 1.5 mm and larger as the required reduction of impurity at manufacturing is not so strict as is required for piano wire.
Accordingly, to the uncoated stress-relieved steel wire and strand for prestressed concrete of JIS G3536 (ASTMA421), the practical tensile strength has been 197 kgf/mm.sup.2 or higher for wire of 2.9 mm diameter, 165 kg/mm.sup.2 or higher for 5 mm diameter, and 189 kgf/mm.sup.2 or higher even for strand wires. Particularly, manufacturing of large diameter strand wires of 12.4 mm, 15.2 mm and 17.5 mm diameters have been difficult as they are made of large diameter wires of 4.2 mm or larger twisted together.
The ropes of large diameter made of two or more wires twisted together require strands of 1.5 mm and larger in most cases, and the toughness is deteriorated by the use of large diameter wire. Accordingly, wires for ropes of over 210 kgf/mm.sup.2 and of over 1.5 mm diameter are not manufactured, which makes the practical application of large diameter high strength rope difficult.
Of the galvanized steel wires for the aluminium cables steel reinforced is specified in JIS C3110 (ASTM B498), those of 2.6 mm diameter with tensile strength of over 180 kgf/mm.sup.2 are produced in large quantity. When the tensile strength exceeds 210 kgf/mm.sup.2, however, the torsional characteristic deteriorates and practical application has not been made possible at the present situation.
When the ordinary high carbon steel wire rod is drawn under the conditions of 8 passes of drawing, 200 m/minute of drawing speed, and 90% reduction in area for example, the torsion value is greatly reduced and the following problems are raised to respective products.
(A) PC wire
At the final taking up of wire after drawing, the wire is broken at the turn roller and the coil straightening roller, thus making the manufacturing impossible. Even if the wire can be manufactured without breakage, the wire is often broken by the anchoring chuck during tensioning at the stage of introducing prestressing force, thus making commercialization impossible.
(B) PC strand
Besides the problem mentioned above, breakage occurs at the stage of stranding if the embrittlement is excessive and thus manufacturing of PC strand is practically impossible. The merit of processing for high strength wire is not obtained because the anchoring efficiency of the strand wire is low due to the brittleness of wire.
(C) Galvanized steel wire
As to the galvanized steel wire for ACSR (aluminium cables steel reinforced) torsion value is specified at the value of more than 16 turns or more than 20 turns. Embrittled steel wires do not meet the specified torsional value due to delamination. As a low torsion value leads to a low fatigue strength it makes commercialization difficult.
(D) Rope
A low torsion value makes stranding impossible. The bending fatigue strength which is an important characteristic for wire rope is also low, and it may lead to serious trouble due to breakage during use.
To prevent embrittlement of steel wires, cold drawing methods are also employed in which the wire, after drawing, is cooled directly with water together with the rear face of the dies to reduce heat generation from the wire at drawing and to cool the wire quickly. For manufacturing of high strength and high toughness wire, however, such methods as the compositions, number of passes of drawing, total reduction in area, patenting, and cold drawing are combined systematically have not been adopted so far.